In modern third generation computed tomography (CT) scanners, an X-ray source and detector array rotate about a subject or object to be scanned. During a scan, the source and detectors image the object at incremental scan angles. A process referred to as reconstruction generates a series of two-dimensional images or slices of the object from the captured data.
The source, detectors, and related components are mounted to a rotatable gantry supported by a rigid stationary frame. As the gantry rotates, a conveyor passing through a central aperture in the gantry translates the object relative to the sensors. In a fixed system, the object remains fixed during each scan; in a translational or "helical" system, the object continuously translates during the scan. In both fixed and translational systems, precision in the angular velocity, or rotation rate, of the gantry is essential for minimization of reconstruction errors.
Timing belts, or cog belts, have been employed in the past to effect a high degree of precision in rotation rate. A standard timing belt is driven by a motor mounted to the stationary frame. Periodic lateral grooves transverse to the major axis of the belt mesh with teeth on a drive sprocket at the motor and a large driven sprocket mounted to the gantry disk. The driven sprocket must be large enough to avoid interference with the central aperture of the gantry and thus allow room for a object to pass therethrough. For this reason, extraordinarily-large timing belts are required in these systems.
A typical prior art scanner requires at least a six meter timing belt. Timing belts of such a large magnitude are very expensive, as they are difficult to manufacture and often must be custom built, and/or purchased in large quantities. Furthermore, the large driven sprockets are specialized and are therefore expensive, available at a cost of $4,000 to $6,000, depending on the diameter. Alignment between the drive sprocket and driven sprocket must be accurate to a high degree of precision, to avoid lateral walking of the belt relative to the sprockets. Timing belts tend to wear rapidly, and therefore must be replaced frequently, for example once per year for a medical scanner. Replacement is an involved procedure, requiring removal of the scanner system from operation for an extended period of time; perhaps a couple of days. This is due to the fact that in prior art configurations, the driven sprocket is positioned between the annular gantry and the fixed frame. Access to the timing belt for its removal and replacement therefore requires complete removal of the gantry from the frame. Positioning of the sprocket on the component side of the gantry is impractical, since the timing belt would interfere with the rotating gantry components.
A further disadvantage of timing belts in CT systems is their tendency to modulate the rotational speed of the gantry at the frequency of their teeth or cogs. The modulation causes artifacts in the resulting images which must be resolved or otherwise corrected by the image processing system.